Create frontend FAQ (documentation)

doc/development/fe_guide/frontend_faq.md

+# Frontend FAQ
+
+## Rules of Frontend FAQ
+
+1. **You talk about Frontend FAQ.**
+   Please share links to it whenever applicable, so more eyes catch when content
+   gets outdated.
+2. **Keep it short and simple.**
+   Whenever an answer needs more than two sentences it does not belong here.
+3. **Provide background when possible.**
+   Linking to relevant source code, issue / epic, or other documentation helps
+   to understand the answer.
+4. **If you see something, do something.**
+   Please remove or update any content that is outdated as soon as you see it.
+
+## FAQ
+
+### How do I find the Rails route for a page?
+
+The easiest way is to type the following in the browser while on the page in
+question:
+
+```javascript
+document.body.dataset.page
+```
+
+Find here the [source code setting the attribute](https://gitlab.com/gitlab-org/gitlab-ce/blob/cc5095edfce2b4d4083a4fb1cdc7c0a1898b9921/app/views/layouts/application.html.haml#L4).

doc/development/fe_guide/index.md

@@ -73,6 +73,10 @@ How we use UI components.
 
 How we use Snowplow to track custom events.
 
+## Frontend FAQ
+
+Read the [frontend's FAQ](frontend_faq.md) for common small pieces of helpful information.
+
 ---
 
 ## Style Guides